screen reader support for chat history

...lin/org/briarproject/briar/desktop/contact/ContactCard.kt

...iarproject/briar/desktop/conversation/ConversationItem.kt

...roject/briar/desktop/conversation/ConversationItemView.kt

...iarproject/briar/desktop/conversation/ConversationList.kt

...briar/desktop/conversation/ConversationMessageItemView.kt

.../briar/desktop/conversation/ConversationNoticeItemView.kt

...briar/desktop/conversation/ConversationRequestItemView.kt

...oject/briar/desktop/conversation/ConversationViewModel.kt

.../org/briarproject/briar/desktop/utils/StringBuilderExt.kt

...esktop/src/main/resources/strings/BriarDesktop.properties